const singer = [{
		name: '周杰伦',
		num: 20
	},
	{
		name: '许嵩',
		num: 25
	},
	{
		name: '林俊杰',
		num: 19
	},
	{
		name: '蔡徐坤',
		num: 10
	},
	{
		name: '鹿晗',
		num: 0
	},
]

Array.prototype.my_find = function(callback) {
	for (let i = 0; i < this.length; i++) {
		if (callback(this[i], i, this)) {
			return this[i]
		}
	}
	return undefined
}

const res = singer.my_find((item, index, arr) => {
	return item.num > 20
})

console.log(res);
